I've found the cheapest flying is to be found by buying a share in a small plane (C150 fpr example). As long as you don't get ciaght with any enormous maintainance bills you should be able to build hours quite cheaply. I own a share in a C150 and it works out a roughly £27/hour all inclusive.
Failing that, I found there were plenty of people out there willing to hire out their planes for a reasonable rate. For example, one guy was offering his Beagle Pup at £50/hr wet.